In vehicles powered by internal combustion engines of the liquid cooled type, it is common practice to circulate engine coolant through an air cooled heat exchanger, or radiator, for cooling the engine. It is also common practice to circulate engine coolant through an air heat exchanger or heater core for providing heated air to the vehicle passenger compartment for cold climate operation of the vehicle.
For many years it has been the practice in automotive vehicle design, to control the flow of coolant to the radiator by retarding or blocking flow by a thermally operated valve, or thermostat, upon cold engine start-up to enable the engine to reach normal operating temperature before the liquid coolant is circulated to the radiator. Such thermostats have heretofore been operated by differentially expansible bi-metal actuators, or more recently, expansible wax pellet charge type actuators for opening the coolant valve upon the coolant in the engine reaching the desired operating temperature. When thermostatic valves of either of the aforementioned types are opened to permit coolant circulation through the radiator, the recirculation of cooled liquid in the engine causes the thermoactuator to severely restrict the flow of coolant through the valve. In particular, it has been found that wax pellet type thermostats operate in cool or cold weather with the thermostatic valve only very slightly open, or almost closed, thereby providing only a fraction of the flow of which the valve is capable of in the fully open position.
When a typical automotive engine coolant thermostat is operated at only a slightly open position, sludge formed by rust and particles of foundry core sand from the engine block casting, have been found to accumulate on the valve and create deposits which thus prevent the thermostat from completely closing. When deposits on the thermostatic valve prevent complete closing thereof, upon cold engine start-up, flow is permitted through the thermostatic valve immediately and warm-up of the engine is thus retarded.
In modern passenger automobile engine design, it has been found that the engine warm-up period must be kept as short as possible in order to reduce the inefficiency of the combustion and the resultant undesirable exhaust emissions resulting from inefficient combustion. Thus, it has been desired to provide control of the liquid coolant in an engine in such a way as to maximize the engine warm-up process.
However, for a given full load cooling capacity of an engine/radiator cooling system, it is necessary to severely restrict or throttle the flow of coolant with the thermostatic valve in order to prevent the engine coolant operating temperature from dropping below the desired level at less than full power or full load operating conditions. In particular, where the radiator has the cooling capacity for accommodating full power vehicle operation in extreme environments such as hot and humid or desert climates at less than full power and in moderate climatic conditions, the thermostat will be required to severely restrict flow to the radiator. This restriction of coolant flow by the thermostatic valve has proven to be troublesome in service because of deposit build-up in the restricted flow position of the valve.
Thus, it has long been desired to find a way or means of controlling engine coolant flow to the radiator in a manner which would maintain constant operating temperature in the face of a widely varying engine power and climatic conditions, and to eliminate the problems encountered with severe throttling through the thermostatically controlled valve.
In another aspect of engine coolant circulation, where the coolant is employed in a heater core for maintaining passenger compartment comfort in cold climate operation, it has been typical automotive design practice to employ a manual control for vehicle occupant selection of the position of a flow control valve, typically of the butterfly-type, for altering the flow of engine coolant through the heater core. In addition, provisions are usually made for the vehicle occupant to select from plural settings of a blower speed control for increasing or decreasing the forced air circulation from the blower over the heater core. Where automatic control of passenger compartment temperature has been desired, typical automotive design practice has been to employ a vacuum motor to vary the position of a blend door for mixing refrigerated air with heated air for controlling the temperature of the air discharged from the blower plenum to the passenger compartment. Such control systems have also employed a blower speed control switch slaved to the vacuum motor for proportioning heater blower speed with the control movements of the air blend door in the plenum.
However, it has long been desired to eliminate mixing heated and refrigerated air to control passenger compartment temperature because this technique requires operation of the air conditioning refrigeration compressor to provide a source of cooled air for tempering the discharge air to the passenger compartment. Thus, it has long been desired to find a way or means of automatically electrically controlling the flow of engine coolant to the heater core in order to enable automatic modulation of the heater core temperature instead of providing refrigerated air from the air conditioning evaporator mixed with the air blown over the heater core in order to provide tempered air to the passenger compartment.